In recent years the rapidly increasing incidence of airplane hijacking, jail breaks and armed attacks on institutions have been possible because there has been no suitable means for detecting the movement of hidden ferrous or non-ferrous metal objects moving through a restricted zone.
Although metal detecting systems have been known for some time, those previously constructed could be defeated by a concerted effort to find a combination of metals and target shapes which would not affect the station. This is because a ferrous metal object moving through the receiving coils of a metal detector would increase the induced voltage in the receiving coils, while a non-ferrous metal object moving through the receiving coils of the metal detector would decrease the induced voltage in the receiving coils. But, in either case the movement of a ferrous or non-ferrous metal object through the station would upset the voltage in the receiving coils and this change could be detected. In addition the shape of the metal object moving through the metal detector in relation to the shape and orientation of the magnetic field set up by the coils is an important factor in determining the effect the metal object has on the voltage in the receiving coils. All this suggests that a properly shaped weapon formed from a suitable combination of ferrous and non-ferrous metals might pass through the metal detecting station undetected. Therefore, an important object of this invention is to provide a metal detecting station for detecting the movement of ferrous or non-ferrous, or combinations of ferrous and non-ferrous metal objects passing through the station, and in particular to provide a metal detecting station which can detect concerted efforts to defeat the station by constructing weapons from a combination of metals and shapes specifically constructed to defeat prior metal detectors.
Another important object of the invention is to provide a metal detecting station for detecting the movement of ferrous or non-ferrous metal objects concealed on persons passing through the station which is not appreciably affected by the movement of ferrous or non-ferrous metal objects moving outside the station. This is important because when a metal detecting station of the type previously known was installed in an airport, bus terminal, or railway station, the movement of the airplanes, busses, or trains could generate a signal in the detecting station which could confuse the operator. Furthermore, if the station were located far enough away from these vehicles, so the station was not affected by their movement, additional problems could be caused by employees or other persons walking near the station while carrying metal objects, such as pails or tools. This is because such metal objects could produce a voltage output in the receiving coils of the station which could confuse the station operator and lead to the defeat of the metal detecting station by persons who understand its weaknesses.
Furthermore, if attempts were made to install the prior detecting systems in penal institutions, similar problems would be encountered if metal doors were opened or closed near the metal detecting station. Although some of these problems could be reduced by carefully positioning the prior metal detecting station, occasional movements of large metal objects within the institution in the area of the metal detecting station would upset the balance of the system. Also, if the metal detecting station was made portable, all the above described problems would constantly recur each time the metal detecting station was shifted, as when the metal detecting station was moved from one gate to another at an airport.
Another object of this invention is to provide a metal detecting station which can be made insensitive to the movements of tiny ferrous or non-ferrous metal objects passing through the station. This is important where the metal detecting station is to be used in public places where the public gathers in large numbers such as at airports, train or bus stations, stadiums and the like. In these areas, it is important that the metal detecting stations be insensitive to harmless tiny metal objects such as metal frames on glasses, rings, keys and the like, to avoid the slowing of passage of people through the metal detecting station to an unacceptable level. This requires a metal detecting station which can be adjusted so as not to respond to these objects and to fully respond to metal objects of sufficient size to constitute a possible weapon.
The metal detecting station constructed according to the principles of this invention has eliminated these difficulties by introducing a fixed bias voltage in the receiving coils and also introducing a substantial phase change between the voltage in the receiving coils and the voltage in the transmitting coil of the metal detecting station such that no usefully shaped combination of ferrous and/or non-ferrous metals above a pre-determined size could move through the station without disturbing the voltage and/or phase angle induced in the receiving coils and thus deceive the station operator. In addition the problem of externally caused magnetic or electromagnetic influences is eliminated by providing the metal detecting station with shielding which prevents these influences from having any appreciable effect on the metal detecting system inside the station. This is accomplished by attaching ferrous sheet metal rigidly to the walls of the metal detecting station. In this way external ferrous or non-ferrous metal objects moving relative to the station have no appreciable effect on the detecting system in the station.